1. Field of the Invention
The present invention relates to a hydraulic circuit for a hydraulic cylinder to be used for a civil engineering machine such as a power shovel or equipment driven by a hydraulic pressure, and more particularly to improvements of a hydraulic circuit for a hydraulic cylinder having a shock absorbing function at the time of stopping the operation of a cylinder piston rod.
2. Description of the Related Art
Conventionally, a hydraulic cylinder is used for various types of civil engineering machines such as a power shovel to drive a working machine such as a bucket.
The hydraulic cylinder is provided with a cylinder rod that performs a linear reciprocating motion within a cylinder by a hydraulic pressure of an operating oil.
The cylinder rod is provided at its one end with a cylinder piston inserted into the cylinder, and the interior of the cylinder is divided into two cylinder chambers by the cylinder piston.
The operating oil is supplied under pressure into one of the two cylinder chambers to linearly move the cylinder rod in an extending direction, and into the other cylinder chamber to linearly move the cylinder rod in a retracting direction, whereby a working machine such as a bucket connected to the end of the cylinder rod is driven to operate.
In order to control the movement of the working machine, there is an occasion in which the hydraulic cylinder stops feeding of the operating oil under pressure to the cylinder chamber by the control valve, whereby the linear movement of the cylinder rod is stopped temporarily.
In such an occasion, if the feeding of the operating oil under pressure to the cylinder chamber is stopped suddenly, the operating oil accumulated in the cylinder chamber is compressed by an inertial force of the cylinder piston. Due to the incompressibility of the operating oil, the operating oil accumulated in the cylinder chamber becomes high resistance so that the cylinder rod stops suddenly. As a result, the cylinder itself receives a large shock, resulting in the generation of large vibration and noise in the equipment using the hydraulic cylinder.
Japanese patent application publication no. 49-104075 discloses the configuration in which an accumulator, which has a shock absorbing function comprising an accumulator piston and a coil spring supporting the accumulator piston, is disposed in a cylinder rod, and both sides of the accumulator piston are constantly communicated with respective corresponding cylinder chambers.
According to the hydraulic cylinder disclosed in Japanese patent application publication no. 49-104075, when the operating oil being fed under pressure to the cylinder chamber is stopped suddenly in order to stop the action of the cylinder rod, the operating oil accumulated in the cylinder chamber is compressed by an inertial force of the cylinder piston connected to the cylinder rod. The operating oil compressed in the cylinder chamber frees in part to the accumulator piston side, and the coil spring is pushed to be extended/retracted by the pressure of the freed portion of the operating oil via the accumulator piston.
Then, the pressure of the operating oil in the cylinder chamber is gradually absorbed by the damper function of the coil spring, which is extended/retracted by the hydraulic pressure of the freed operating oil, until it balances with the pressure due to the coil spring. Thus, the shock generated in the cylinder is reduced.
Incidentally, in the conventional hydraulic cylinder that has the accumulator having the shock absorbing function disposed in the cylinder rod as described in Japanese patent application publication no. 49-104075, the accumulator is constituted by an accumulator piston and a coil spring that supports the accumulator piston, and the both sides of the accumulator piston are kept communicated with their corresponding cylinder chambers through oil passages, so that the shock absorbing function is constantly acting on the hydraulic cylinder.
Therefore, with the conventional hydraulic cylinder, when the cylinder rod is stopped suddenly to quickly stop the working machine at a prescribed position, the cylinder rod is kept vibrating until the hydraulic pressure in the cylinder chamber and the pressure of the spring of the accumulator are balanced with each other by the shock absorbing function of the accumulator that keeps acting. Thus, the conventional hydraulic cylinder has a disadvantage that the working machine such as a bucket cannot be stopped and positioned quickly at a prescribed position by suddenly stopping the cylinder rod.